A typical molten metal facility includes a furnace with one or more pumps for moving molten metal. During the processing of molten metals, such as aluminum, the molten metal is normally continuously circulated through the furnace by a centrifugal impeller pump, i.e., a circulation pump, to equalize the temperature of the molten bath. A typical furnace includes a pump well that is located between the heating chamber or hearth and the charge well (where raw material is inserted into the furnace). These three main sections of a typical furnace are fluidly interconnected with the circulation pump causing the molten metal to circulate from the pump well to the charge well to the hearth and back into the pump well.
To transfer the molten metal out of the furnace, typically for casting the metal, a second transfer pump is used to elevate the metal up through a discharge conduit that runs up and out of the furnace. Once the furnace is empty or near empty, the transfer pump is shut down and new solid material is placed within the furnace to be melted.
Melting and transferring the metal in batches requires a significant amount of energy. This is particularly so due to the latent heat of fusion at the melting point. For example, the latent heat of fusion for aluminum is approximately 171 Btu/lb (397 kJ/kg). For a typical-sized furnace, e.g., having a 10,000 to 200,000 pound capacity, the amount of heat required to melt the next batch of aluminum would require between approximately 1.7 million to 34 million Btu just to raise the metal one degree Celsius at its melting point and cause the metal to change from solid to liquid.
To insure that the metal remains liquid after it is removed from the furnace and while it is transferred to the final casting processes/molds, the metal is normally heated far beyond its melting point (e.g., to approximately 1400° Celsius for aluminum, which has a melting point of approximately 66° Celsius).
One drawback of this traditional batch transfer process is that removing all of the molten metal from the furnace also removes all of the potential heat energy contained within the molten metal. In a continuous casting process, some of the molten metal is continually transferred out of the furnace to the molds, while most of the molten metal is re-circulated throughout the furnace. The heat energy of the molten metal is used to melt the solid raw material inserted into the furnace to replenish the molten metal being continually poured out.
In a molten metal re-circulating furnace, the pump well typically includes the circulation pump and, if there is enough space, a separate transfer pump. If there is not enough room in a particular furnace's pump well, the circulation pump must be removed and replaced with the transfer pump when retrieving/pouring molten metal from the furnace. While the current two pump system can be effective for its intended purpose, purchasing and maintaining two separate pumps is expensive and inefficient, particularly when the two pumps must be swapped out in smaller pump wells. Swapping out furnace pumps also prevents the furnace operator from utilizing a continuous casting process.
In a typical furnace, additional materials such as chlorine and nitrogen gas are injected into the metal at the pump outlet. The gas injection, however, causes both dross formation and violent shaking and vibrations in the immediate area where the injection occurs due to the rapid expansion of the gas. Further, it is well known that increasing the metal's flow rate facilitates gas injection into the molten metal. Typically, the pump velocity is increased briefly to increase the flow rate while injecting the gas, but running the pump at a higher than normal rate increases the risk of damaging the pump. This is particularly so when chlorine is being injected into the metal (aluminum) forming magnesium chloride dross.
There is therefore a need for a device that can be immersed in a bath of molten metal that can selectively redirect at least a portion of the output from the furnace's pump out of the furnace and which can control the flow rate of the re-circulating molten metal.